The trouble is, we don't know if what the boy says happened is actually what did happen. It could be that he has exaggerated it significantly.
I have personal experience of this. Several years ago, I saw and heard a parent march angrily to the school office, demanding to speak to the head. All guns blazing, she demanded to speak to the head immediately, was shouting how dare a teacher smack her daughter on the head, didn't she know it was illegal. She had rounded up a couple of other children with their parents parents at the school gate who had 'witnessed' this incident, basically approached two other children who had been in the classroom at the time and said 'you saw it happen, didn't you?'- she was so loud and angry that the children would probably have been scared to say no! This scene at the school gate was witnessed by another teaching colleague.
What had actually happened? The girl (aged 8) had been chatting instead of working, and the teacher walked past her on the way to another table, and with a single sheet of A4 paper very gently tapped her on the top of the head as she walked past and said "time to stop chatting and get on with your work". Yes, it's true that she shouldn't have done it, but it's also true that a single sheet of paper is not the same as smacking a child on the head.
How do I know that was what actually happened? Because I just happened to be in the room at the same time. When the witness children were later interviewed as well, they were able to give same version of events as I did, without influencing them the same way that the angry parent had with leading questions.
That colleague's 30 year teaching career could have been ended as a result of that one incident. Just shows how children sometimes (though not always) do exaggerate things, not always deliberately. Sometimes if they feel upset then in their minds things were worse than they actually were.
The school should investigate the matter formally, you should insist on that with the head. But you shouldn't have approached the TA when you were angry, that's not the way to sort out this situation.